Tunable wavelength light emitting diode

ABSTRACT

A light emitting diode and a method of fabricating a light emitting diode, the diode has a first set of multiple quantum wells (MQWs), each of the MQWs of the first set comprising a wetting layer providing nucleation sites for quantum dots (QDs) or QD-like structures in a well layer of said each MQW; and a second set of MQWs, each of the MQWs of the second set formed so as to exhibit a photoluminescence (PL) peak wavelength shifted compared to the MQWs of the first set.

FIELD OF INVENTION

The present invention relates broadly to a light emitting diode and to amethod of fabricating a light emitting diode.

BACKGROUND

Light emitting diode (LEDs) have been used in many applications such asoutdoor full colour displays, traffic lights, data storage, solid statelighting and communications. Presently, LEDs can only emit light at aparticular wavelength. White LEDs are made up of three separate LEDsemitting light with the three primary colours of blue, green and red.Conventional diodes are made from inorganic compound semiconductors,typically AlGaAs (red), AlInGaP (orange-yellow-green) and InGaN(green-blue). These diodes emit monochromatic light of a frequencycorresponding to the bandgap of the compound semiconductor. Thedifference in the degradation time of these different materials cancause the problem in the extent of white obtained over time. This alsoapplies for the phosphor-based white LEDs, where the different rates ofdeterioration of the phosphors makes the lifetime for which the devicecan generate white light shorter than the lifetime of the devicesitself. An additional problem with this approach include a low emissionefficiency, stock losses and complex packaging as a phosphor layer hasto be incorporated into the devices, which leads to a non-reliability ofthe LEDs. In full color displays, LEDs are used in backlighting and itis essential that the LEDs emit light with a constant ratio of intensityfor the respective component wavelengths.

In phosphor based LEDs, phosphor coating can be used to convert blueLEDs to light over a wider spectrum, typically yellow. The combinationof yellow and blue light enables the emission of white light.Alternatively, a multi-phosphor blend can be used to generate light suchas trichromatic red-green-blue (RGB). However, the extent of the yellow,green or cyan is not tunable as the phosphor can emit light only at aparticular wavelength. Furthermore, the approach is expensive andcomplex since each of the blue, green and red LEDs has to be addressedindependently and a feedback is needed.

Amongst LEDs, group III-nitride based LEDs have attracted considerableinterest in the field of optoelectronics as their bandgap varies andcover a wide range of emission spectra from ultraviolet to infra-redutilising binary and ternary alloys e.g. AlN, Al_(x)Ga_(1-x)N,In_(y)Ga_(1-y)N and InN. InGaN/GaN multiple quantum wells (MQWs) areoften employed in the active regions of the group III-nitride based LEDsand laser diodes (LDs). However, the epitaxial growth of InGaN/GaN MQWsposes a great challenge, especially when high In content has to beincorporated for long wavelength applications such as green or red LEDs.Furthermore, the light output efficiency tends to be lowered for lightemission with increasing wavelength or higher In incorporation. Loweringthe growth temperature results in the increase in the incorporation ofIn but a reduction in the Photoluminescence (PL) intensity as thecrystalline quality is degraded.

Recently, Indium quantum dots have been explored by Chua et al. [Soo JinChua et al. US 2004/0023427 A1, Pub Date: Feb. 5, 2004] to achieve ared-shift in the PL emission. Indium Nitride (InN) and Indium-richIndium Gallium Nitride (InGaN) quantum dots embedded in a single and inmultiple I_(x)Ga_(1-x)N/In_(y)Ga_(1-y)N quantum wells (QWs) were formedby using trimethylIndium (TMIn) as antisurfactant during MOCVD growth,and the photoluminescence wavelength has been shifted from 480 to 530 nm[J. Zhang et. al. Appl. Phys. Lett. v80, p 485-487, 2002]. However, thegrowth of the LEDs using such a technique gives only green emission fromthe MQWs. There is currently no possibility of getting a red emissionfrom InGaN/GaN MQWs. Perez-Solorzano and co-workers [Perez-Solorzano etal. Appl. Phys. Lett. v87, p 163121-1, 2005] have reported on near-redemission from site controlled pyramidal InGaN quantum dots (QDs),however there is no report on a GaN based LED giving red emission.Practical visible red-orange and yellow light sources have been achievedusing AlInGaP, while bright green, blue and violet LEDs are fabricatedfrom GaN based material system. However, even though these diodes, whenadded together, give a full color display with sufficient brightness,there is no single MQW structure which can emit light with tunablewavelength.

US Patent application publication US 2005/0082543 discloses fabricationof low defect nanostructures of wide bandgap materials andoptoelectronics devices. A nanolithographically-defined template isutilised for formation of nanostructures of wide bandgap materials andhas been used for fabrication of phosphor-less monolithic white lightemitting diodes. The fabrication involves the tuning of the size of theQDs to generate light of different wavelengths. White light iscollectively generated by mixing of QDs sized to generate 30% red light,59% green light and 11% blue light. The nano-pattern substrate includesthe use of SiO₂ or other pattern masks using lithography techniques.Thus, the fabrication requires a special template to generate theformation of the QDS pattern to give the different colour emission,which increases the complexity and cost of the resulting LEDs.

US Patent application publication US 2003/127660 A1 discloses anelectronic device which comprises of QDs embedded in a host matrix and aprimary light source which causes the dots to emit secondary light of aselected colour. The host matrix consists of a solid transparentprepolymer colloid and the quantum dots of varying size distribution.The quantum dot consists of materials such as ZnS ZnSe, CdSe and CdS. Asolid state light source, for instance, is used to illuminate the dotscausing them to photoluminescence light of a colour characteristic oftheir size distribution. The light may be pure colour (corresponding toa monodisperse size distribution of quantum dots) or mixed colour(corresponding to a polydisperse size distribution of quantum dots).However, again the fabrication requires a special “template”, here ahost matrix of polymer. Furthermore, the fabrication requires theimplementation of QDs which use foreign materials, which may quench theluminescence. Thus, this fabrication technique is complex and thusincreases cost of the LEDs, with potentially quenched luminescence.

A need therefore exist to provide a light emitting device that seeks toaddress at least one of the above problems.

SUMMARY

In accordance with a first aspect of the present invention there isprovided a light emitting diode comprising a first set of multiplequantum wells (MQWs), each of the MQWs of the first set comprising awetting layer providing nucleation sites for quantum dots (QDs) orQD-like structures in a well layer of said each MQW; and a second set ofMQWs, each of the MQWs of the second set formed so as to exhibit aphotoluminescence (PL) peak wavelength shifted compared to the MQWs ofthe first set.

The QDs or QD-like structures may comprise In atoms.

The first set of MQWs may comprise about 3 to 5 MQWs.

The second set of MQWs may comprise about 2 to 5 MQWs.

Each of the MQWs of the second set may comprise a Ga-based barrier layerand an Ga-based well layer formed on the Ga-based barrier layer.

Each of the MQWs of the first set may comprise a Ga-based barrier layer,an InGa-based wetting layer formed on the Ga-based barrier layer, and aGa-based well layer formed on the InGa-wetting layer.

The first set of MQWs may be formed on an n-type doped Ga-based layer,the second set of MQWs is formed on the first set of MQWs, a Ga-basedcapped layer is formed on the second set of MQWs, and a p-type dopedGa-based layer is formed on the Ga-based capped layer.

The light emitting diode may further comprise electrical contacts forcontacting the n-type doped Ga-based layer and the p-type doped Ga-basedlayer respectively.

The first and second sets of MQWs may be supported on a substrate.

The MQWs may comprise one material system of a group consisting ofInGa/Ga, InGa/AlGa, Ga/AlGa and InGa/AlInGa.

The MQWs may comprise the nitride or phosphide of the material system.

A combined PL spectrum of the diode may cover a tunable wavelength rangeof about 400-800 nm.

In accordance with a second aspect of the present invention there isprovided a method of fabricating a light emitting diode, the methodcomprising the steps of forming a first set of multiple quantum wells(MQWs), each of the MQWs of the first set comprising a wetting layerproviding nucleation sites for quantum dots (QDs) or QD-like structuresin a well layer of said each MQW; and forming a second set of MQWs, eachof the MQWs of the second set formed so as to exhibit aphotoluminescence (PL) peak wavelength shifted compared to the MQWs ofthe first set.

The QDs or QD-like structures may comprise In atoms.

The first set of MQWs may comprise about 3 to 5 MQWs.

The second set of MQWs may comprise about 2 to 5 MQWs.

Each of the MQWs of the second set may comprise a Ga-based barrier layerand an InGa-based well layer formed on the Ga-based barrier layer.

Each of the MQWs of the first set may comprise a Ga-based barrier layer,an InGa-based wetting layer formed on the Ga-based barrier layer, and aGa-based well layer formed on the InGa-wetting layer.

The first set of MQWs may be formed on an n-type doped Ga-based layer,the second set of MQWs is formed on the first set of MQWs, a Ga-basedcapped layer is formed on the second set of MQWs, and a p-type dopedGa-based layer is formed on the Ga-based capped layer.

The method may further comprise forming electrical contacts forcontacting the n-type doped Ga-based layer and the p-type doped Ga-basedlayer respectively.

The first and second sets of MQWs may be supported on a substrate.

The first set of MQWs may be formed at a lower temperature than thesecond set of MQWs.

The first set of MQWs may be formed with a higher In precursor flow thanthe second set of MQWs.

The MQWs may comprise one material system of a group consisting ofInGa/Ga, InGa/AlGa, Ga/AlGa and InGa/AlInGa.

The MQWs may comprise the nitride or phosphide of the material system.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readilyapparent to one of ordinary skill in the art from the following writtendescription, by way of example only, and in conjunction with thedrawings, in which:

FIG. 1 shows a schematic cross-sectional view of the sample structure ofan LED according to an embodiment.

FIG. 2 shows an SEM image of the surface morphology of the p-type InGaNin the LED of FIG. 1.

FIGS. 3 to 15 show schematic cross sectional drawings illustratingfabrication of a light emitting device according to an embodiment.

FIG. 16 shows the I-V characteristic of an LED according to anembodiment.

FIG. 17 is a graph showing the dual PL peak emission spectrum from theLED of FIG. 16.

FIG. 18 is a cross-sectional TEM image showing the dual set of MQWs inthe LED of FIG. 16.

FIG. 19 shows the colour coordinate of the emission at differentvoltages on the chromaticity Diagram, CIE, for the LED of FIG. 16. Thefollowing points correspond to the following voltage range; Point (A)

: 3-4V, Point (B)

: 5-7V, Point (C)

: 8-10V, Point (D)

: 11-20V.

DETAILED DESCRIPTION

The growth of the LEDs in the example embodiments was performed using ametalorganic chemical vapour deposition (MOCVD) system. Trimethylgallium(TMGa), Trimethlyindium (TMin), Trimethylaluminium (TMA), Magnesium(Cp₂Mg) and silane (SiH₄) were used as the precursors. Hydrogen andnitrogen were used as the carrier gas for effective incorporation of theelements.

In the LEDs, two sets of MQWs (compare 100, 102 in FIG. 1) were grown atdifferent temperatures so as to obtain emissions at differentwavelengths. After a high temperature GaN layer (compare layer 3 in FIG.1), grown on a low temperature GaN buffer (compare layer 2 in FIG. 1) one.g. a sapphire substrate (compare layer 1 of FIG. 1), the temperaturein the MOCVD chamber is lowered to about 700-750° C. to grow a first setof MQWs consisting of about 3 to 5 wells. A GaN barrier (compare layer 4in FIG. 1) is first grown to a thickness of about 5.0-10.0 nm with Sidoping, n_(s) about 2.0×10¹⁷cm⁻³. A thin wetting layer (compare layer 5in FIG. 1) of In_(x)Ga_(1-x)N with composition of x about 0.10-0.20 andthickness of about 1 nm is grown to enhance the incorporation of theIndium Nitride rich QDs during the In burst process. The In atoms fromthe Indium precursor segregate at the dangling bonds of the wettinglayer of InGaN to serve as a seed layer for the subsequent growth of theInGaN QDs and well layer (compare layer 6 in FIG. 1) as determined bythe precursor flow rate. The amount of TMIn acting as antisurfactantsand the duration of the TMIn flow are important for the growth of theIndium rich QDs. It was found that too small a flow may not form enoughseeds for growth of the QDs, but too long a duration may causes welllayer roughening.

After the growth of the first set of MQWs, an about 10-30 nm undoped GaNlayer (compare layer 7 in FIG. 1) is grown at about 720-750° C. beforethe temperature is increased by another about 30° C. for the growth ofthe second set of MQWs. A n-type GaN barrier (compare layer 8 in FIG. 1)is grown to a thickness of about 5.0-10.0 nm and the InGaN (comparelayer 9 in FIG. 1) well is grown to a thickness of about 2.0-5.0 nm. TheTMIn flow during the growth of the second set of MQWs is lowered toabout 300 sccm or 43.0 μmol/min based on the vapour pressure and thetemperature of the TMI source. It was found that the lower TMI flow rategives a blue-shift in PL emission. The 2^(nd) set of MQWs consists ofabout 2 to 5 wells.

After the growth of the 2^(nd) set of MQWs, a thin capped layer (comparelayer 10 in FIG. 1) of GaN is grown to a thickness of about 15-30 nm atabout 780-800° C. Next, a layer of Al_(a)Ga_(1-a)N (compare layer 11 inFIG. 1) of thickness 20-40 nm is grown, where a is between about0.1-0.3. This is followed by a p-type InGaN layer (compare layer 12 inFIG. 1) grown to a thickness of about 150-300 nm. Magnesium is used asthe p-dopant and growth in the chamber is carried out at about 750-800°C. The TMIn flow rate is set in the range of about 80-150 sccm with apressure less than about 300 Torr. The pressure is subsequently loweredto about 50-300 Torr for the growth of a thin epilayer of Mg-doped GaNin hydrogen ambient. This was found to improve on the contactresistance. In order to prevent the out-diffusion of the In-rich InGaNnanostructure in the well layer (compare layer 6 of FIG. 1) of the firstset of MQWs, a conventional p-type GaN is doped with Indium and thegrowth of the p-type InGaN (compare layer 12 in FIG. 1) is kept in therange of about 750-800° C. No additional in-situ annealing is carriedout to activate the Mg in the p-type InGaN (compare layer 12 in FIG. 1).

FIG. 1 shows a schematic cross-sectional view of the dual set ofInGaN/GaN MQWs 100, 102 structure of the example embodiment. Layer 1 isthe substrate which can be sapphire, Silicon carbide (SiC), zinc oxide(ZnO) or other substrate. Layer 2 is the low temperature GaN buffergrown at 500-550° C. with a thickness of about 25 nm to facilitate thenucleation of GaN on the sapphire substrate. Layer 3 is the Si-dopedhigh temperature GaN layer grown at around 1000-1050° C., doped to aconcentration of about 2×10¹⁷ to 9×10¹⁸ cm⁻³. Layer 4 to Layer 6 is thefirst set of MQWs. Layer 4 is the Si-doped GaN barrier with a dopingconcentration of about 2×10¹⁷ to 2×10¹⁸ cm⁻³. Layer 5 is the wettinglayer of In_(x)Ga_(1-x)N pre-growth before TMIn burst. The Indiumcontent, x, ranges from about 0.1 to 0.2. After the growth of layer 5,TMIn and ammonia were flowed to form the seeds for the growth of Indiumrich QDs before layer 6 is deposited. The flow rate is maintained atabout 10-80 μmol/min for duration of about 3 to 12 seconds while thetemperature in the chamber is ramped down by about 10° C. during theTMIn flow. Layer 6 is the In_(y)Ga_(1-y)N well layer with the embeddedIndium rich nanostructure 104, where y>x. The embedded In richnanostructure 104 has an Indium content ranging from about 10 to 60% andemits light in the longer wavelength. Layer 7 is an undoped GaN cappedlayer of about 15-30 nm grown at about 720-750° C. Layers 8-9 representthe 2^(nd) set of MQWs, where the temperature has been increased byabout 30° C. from that of the 1^(st) set of MQWs. The composition ofIndium in the 2^(nd) set of MQWs, In_(z)Ga_(1-z)N, has the molefractions z<y as compared to the 1^(st) set of MQWs withIn_(y)Ga_(1-y)N. Layer 10 is the thin capped layer of low temperatureGaN of about 15-30 nm at about 780-800° C. Layer 11 is a layer ofAl_(a)Ga_(1-a)N of thickness 20-40 nm, where a is between about 0.1-0.3.The layer 12 is the Mg doped (p-type) In_(m)Ga_(1-m)N layer where m isbetween about 0.05-0.1.

FIG. 2 shows a scanning electron microscopy (SEM) image of the surfacemorphology 200 of the p-type InGaN layer 12. The surface morphology 200appears to be porous.

Processing is then carried out for the Schottky and the ohmic contactson the p-type InGaN (layer 12, FIG. 1) and the n-type GaN (layer 3,FIG. 1) respectively. A more detailed description of the fabricationprocess will now be given with reference to FIGS. 3 to 15.

With reference to FIG. 3, the method for fabricating a device that emitslights from red to cyan with varying voltages in one example embodimentcomprises providing a substrate 300 with an epilayer 302 of GaN bufferlayer and a n-type GaN layer 304. The epilayer 302 consists of thegrowth of a nucleation (buffer) layer of GaN at about 500-550° C. whilethe n-type GaN layer 304 is grown at 1000-1050° C.

Next, the substrate 300 is maintained at about the same temperature anda layer 400 of Si-doped GaN is deposited with a doping concentration ofabout 2×10¹⁷ to 1×10¹⁸ cm⁻³, as shown in FIG. 4.

The substrate 300 is then maintained at about the same temperature and alayer 500 of In_(x)Ga_(1-x)N, where x ranges from about 0.10 to 0.20, isformed to serve as the wetting layer over the GaN layer 400. With thesubstrate 300 temperature decreased by about 10° C. and flowingindium-precursor at a flow rate of about 10-80 μmol/min between about 3to 12 seconds, a quantum dots-like structure 600 of In_(w)Ga_(1-w)N,where 0.2<w <1.0, is formed, as shown in FIG. 6.

Next, maintaining the substrate 300 at the stable temperature attainedin growth of quantum dots-like structure 600 of In_(w)Ga_(1-w)N, a welllayer 700 of In_(y)Ga_(1-y)N, where y>x, is formed, as shown in FIG. 7.The steps described with reference to FIGS. 4 to 7 are repeated threetimes to form MQWs structures 800, 802, 804 as shown in FIG. 8, whereeach of the structures 800, 802, 804 comprises a layer 400, a layer 500,a quantum dots-like structure 600 and a well layer 700 (FIGS. 4 to 7).

Next, the substrate 300 is maintained at the same temperature as thatfor the step described above with reference to FIG. 3 of about 700° C.to 850° C. and a capped layer 900 of GaN is deposited, as shown in FIG.9. The substrate 300 temperature is then increased by 30° C. and a layer1000 of Si-doped GaN with a doping concentration of about 2×10¹⁷ to1×10¹⁸ cm⁻³ is deposited, as shown in FIG. 10. The substrate ismaintained at about the same temperature and a well layer 1100 ofIn_(z)Ga_(1-z)N, where z<y, is formed, as shown in FIG. 11. The stepsdescribed with reference to FIGS. 10 and 11 are repeated two times toform MQWs structures 1200, 1202 as shown in FIG. 12. Each of thestructures 1200, 1202 comprises a Si-doped GaN layer 1000 and a welllayer 1100 (FIGS. 10, 11).

The substrate 300 is maintained at about the same temperature and acapped layer 1300 of Si-doped GaN with a doping concentration of about2×10¹⁷ to 1×10¹⁸ cm⁻³ is deposited, as shown in FIG. 13. The substratetemperature is then increased by about 30° C. and a layer 1400 ofAl_(x)Ga_(1-x)N where 0.1<x<0.3 is formed, as shown in FIG. 14. Thesubstrate is maintained at about the same temperature and a capped layer1500 of p-type In_(m)Ga_(1-m)N with Mg doping of about 1×10¹⁸ to 1×10¹⁹cm⁻³ is deposited, where m is between about 0.05 to 0.10, as shown inFIG. 15.

Processing is then carried out to make the contact for the devices.Etching of mesa is carried out using BCl₃ and Cl₂ plasma to reach layer3 of FIG. 1 (layer 304 of FIG. 3) using Inductive Coupled Plasma Etching(ICP). The p-contact to p-type InGaN layer 1500 followed by n-contact ton-type GaN are then deposited.

The described embodiments seek to produce a single GaN-based LED packagechip which can give different colour emission by varying the appliedvoltage. In terms of the quality of the LEDs, the embodiments canovercome the problem of difference in lifetime of the set of LEDs used,especially when applied for white LEDs or in automobile indicatorlighting. As the GaN-based LEDs make use of the quantum dots in thequantum well to produce the emission of the desired wavelength, theproblem of degradation and difficulty of packaging associated with theuse of phosphor to achieve the different colour emissions can beresolved. The embodiment also avoids the use of polychromatic colourmixing devices to achieve the desired emission wavelength since theemission wavelength itself is tunable by varying the voltages applied tothe diode.

FIG. 16 shows the I-V characteristic for one example LED. In thatembodiment, In rich InGaN nanostructures are grown on aIn_(0.10)Ga_(0.90)N wetting layer using trimethylIndium (TMIn) burstduring the growth of InGaN/GaN MQWs. These In rich InGaN nanostructuresformed using TMIn as the antisurfactant range from about 50-80 nm,serving as “QDs-like” states in the structure which act as a nucleationsite to enhance In incorporation in the InGaN well layer. This givesrise to a broad PL emission peak 1700 with wavelength in the range of500-700 nm, as shown in FIG. 17. The embedded layer of InGaNnanostructures 1800 can be observed from the transmission electronmicroscopy (TEM) image in FIG. 18. In order to produce emission withdifferent wavelength, two sets of MQWs are implemented. The 1^(st) setof MQWs gives a broad emission band 1700 from about 500-700 nm and2^(nd) set of MQWs enables an emission peak 1702 at 460 nm, as shown inFIG. 17. FIG. 19 shows the colour coordinates of the emission of the LEDat different voltages on the chromaticity diagram, CIE (InternationalCommission on Illumination) in particular (a) 3-4V, (b) 5-7V, (c) 8-10V,and (d) 11-20V.

The ability to fabricate LEDs with tunable colours as the voltage isvaried is useful for a number of important applications, including:

-   -   Illumination and display purposes. This includes the        illumination of signboard, displays in shops, houses and the        sidewalk.    -   LCD backlighting, keypad light guides, digital camera flash        light, PC monitors backlighting. The use of LEDs, especially the        Rainbow LEDs will further enhance the capability of the LEDs to        provide colour rendering.    -   Solid state lighting.    -   Automobile headlights and traffic lights.

It will be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

It is noted that the schematic drawings in FIGS. 1 and 3 to 15 are notto scale. The present invention can also be applied to cover othermaterials such as the phosphide based emitting devices, which includeIn/Ga, InGa/AlGa, Ga/AlGa and InGa/AlInGa (well/barrier) system baseddevices. It will be appreciated that the colors as a function of voltageand the overall wavelength ranges of the devices may vary betweendifferent materials. The wetting layer used is dependent on the elementsin the well layer. The same ternay or quaternary alloy can be used, forinstance an In_(x)Ga_(1-x)P wetting layer is adopted for anIn_(y)Ga_(1-y)P well layer, with x<y. The elements to be incorporatedfor QDs generation are also determined by the elements in the welllayer. In atoms are incorporated in the described example as In can givethe necessary red shift and a broad spectrum for nitride material. Inthe case of GaP, the InP QDs used can enhance its emission in the redregime to infra-red.

The invention claimed is:
 1. A tunable light emitting diode comprising:a first set of multiple quantum wells (MQWs), each of the MQWs of thefirst set comprising a well layer having quantum dots (QDs) formedtherein, and a wetting layer having a different composition than thequantum dots and providing nucleation sites for the QDs in the welllayer of said each MQW; and a second set of MQWs, each of the MQWs ofthe second set formed so as to exhibit a photoluminescence (PL) peakwavelength shifted compared to the MQWs of the first set, wherein theQDs comprise In atoms, and the first set of MQWs is configured toprovide a broad emission band with wavelength at least in the range of500 nm to 700 nm, and wherein the tunable light emitting diode furthercomprises electrical contacts for receiving an applied voltage tocontrol an output wavelength of the light emitting diode such that theoutput wavelength of the light emitting diode is tunable by varying theapplied voltage.
 2. The light emitting diode as claimed in claim 1,wherein the QDs comprise In atoms.
 3. The light emitting diode asclaimed in claim 1, wherein the first set of MQWs comprises about 3 to 5MQWs.
 4. The light emitting diode as claimed in claim 1, wherein thesecond set of MQWs comprises about 2 to 5 MQWs.
 5. The light emittingdiode as claimed in claim 1, wherein each of the MQWs of the second setcomprises a Ga-based barrier layer and an Ga-based well layer formed onthe Ga-based barrier layer.
 6. The light emitting diode as claimed inclaim 1, wherein each of the MQWs of the first set comprises a Ga-basedbarrier layer, an InGa-based wetting layer formed on the Ga-basedbarrier layer, and a Ga-based well layer formed on the InGa-wettinglayer.
 7. The light emitting diode as claimed in claim 6, wherein thefirst set of MQWs are formed on an n-type doped Ga-based layer, thesecond set of MQWs is formed on the first set of MQWs, a Ga-based cappedlayer is formed on the second set of MQWs, and a p-type doped Ga-basedlayer is formed on the Ga-based capped layer.
 8. The light emittingdiode as claimed in claim 7, further comprising electrical contacts forcontacting the n-type doped Ga-based layer and the p-type doped Ga-basedlayer respectively.
 9. The light emitting diode as claimed in claim 1,wherein the first and second sets of MQWs are supported on a substrate.10. The light emitting diode as claimed in claim 1, wherein the firstand second sets of MQWs comprise one material system of a groupconsisting of InGa/Ga, InGa/AlGa, Ga/AlGa and InGa/AlInGa.
 11. The lightemitting diode as claimed in claim 10, wherein the MQWs comprise thenitride or phosphide of the material system.
 12. The light emittingdiode as claimed in claim 1, wherein a combined PL spectrum of the diodecovers a tunable wavelength range of about 400-800 nm.
 13. A tunablemethod of fabricating a light emitting diode, the method comprising thesteps of: forming a first set of multiple quantum wells (MQWs), each ofthe MQWs of the first set comprising a well layer having quantum dots(QDs) formed therein, and a wetting layer having a different compositionthan the quantum dots and providing nucleation sites for the QDs in thewell layer of said each MQW; and forming a second set of MQWs, each ofthe MQWs of the second set formed so as to exhibit a photoluminescence(PL) peak wavelength shifted compared to the MQWs of the first set,wherein the QDs comprise In atoms, and the first set of MQWs isconfigured to provide a broad emission band with wavelength at least inthe range of 500 nm to 700 nm, and wherein the tunable light emittingdiode further comprises electrical contacts for receiving an appliedvoltage to control an output wavelength of the light emitting diode suchthat the output wavelength of the light emitting diode is tunable byvarying the applied voltage.
 14. The method as claimed in claim 13,wherein the QDs comprise In atoms.
 15. The method as claimed in claim13, wherein the first set of MQWs comprises about 3 to 5 MQWs.
 16. Themethod as claimed in claim 13, wherein the second set of MQWs comprisesabout 2 to 5 MQWs.
 17. The method as claimed in claim 13, wherein eachof the MQWs of the second set comprises a Ga-based barrier layer and anInGa-based well layer formed on the Ga-based barrier layer.
 18. Themethod as claimed in claim 13, wherein each of the MQWs of the first setcomprises a Ga-based barrier layer, an InGa-based wetting layer formedon the Ga-based barrier layer, and a Ga-based well layer formed on theInGa-wetting layer.
 19. The method as claimed in claim 18, wherein thefirst set of MQWs are formed on an n-type doped Ga-based layer, thesecond set of MQWs is formed on the first set of MQWs, a Ga-based cappedlayer is formed on the second set of MQWs, and a p-type doped Ga-basedlayer is formed on the Ga-based capped layer.
 20. The method as claimedin claim 19, further comprising forming electrical contacts forcontacting the n-type doped Ga-based layer and the p-type doped Ga-basedlayer respectively.
 21. The method as claimed in claim 13, wherein thefirst and second sets of MQWs are supported on a substrate.
 22. Themethod as claimed in claim 13, wherein the first set of MQWs is formedat a lower temperature than the second set of MQWs.
 23. The method asclaimed in claim 13, wherein the first set of MQWs is formed with ahigher In precursor flow than the second set of MQWs.
 24. The method asclaimed in claim 13, wherein the first and second sets of MQWs compriseone material system of a group consisting of InGa/Ga, InGa/AlGa, Ga/AlGaand InGa/AlInGa.
 25. The method as claimed in claim 24, wherein the MQWscomprise the nitride or phosphide of the material system.
 26. The lightemitting diode as claimed in claim 1, wherein the MQWs are formed at atemperature of at least 700°.
 27. The method as claimed in claim 13,wherein the MQWs are formed at a temperature of at least 700°.